Ammonia synthesis with catalyst derived by heating on a support a salt selected from alkali metal, alkaline earth metal, iron and cobalt hexacyanocobaltates and hexacyanoruthenates

ABSTRACT

Synthesis of ammonia employing at least one salt selected from alkali metal-, an alkaline earth metal-, iron- and cobalt hexacyanocobaltate and hexacyanoruthenate on a suitable support and then heating the composition to provide it with suitable activity.

BRIEF SUMMARY OF THE INVENTION

Ammonia is synthesized at relatively lower temperature employing acatalyst prepared by adding to a suitable support a salt selected fromat least one of an alkali metal, an alkaline earth metal, iron andcobalt hexacyanocobaltates and hexacyanoruthenates and then heating toactivate a composition thus obtained to provide it with suitableactivity to catalyze the synthesis.

DETAILED DESCRIPTION

This invention relates to the synthesis of ammonia. It also relates tothe provision of a catalyst for the synthesis of ammonia. In one of itsaspects the invention relates to the synthesis of ammonia at arelatively lower temperature as herein further described.

In one of its concepts the invention provides a process for ammoniasynthesis which comprises subjecting nitrogen and hydrogen under ammoniasynthesis condition to the action of a catalyst as herein described. Inanother of its concepts the invention provides the catalyst obtained byactivating upon a suitable support a salt selected from at least one ofan alkali metal, an alkaline earth metal, iron and cobalthexacyanocobaltates and hexacyanoruthenates. A further concept of theinvention provides an ammonia synthesis, employing a catalyst as hereindescribed, conducted at a temperature in the approximate range of about385° C. to about 460° C., although the catalyst is active for thesynthesis at both substantially lower and higher temperatures, (about725° F. to about 860° F.) e.g., from about 260° C. to about 590° C.

Current Ammonia Synthesis Technology uses a fused iron-alkali oxidecatalyst, pressures from 100 to 1000 atmospheres, and temperatures about900° F. to 950° F. Even with these extreme conditions, the per passconversions are only 8 to 20 percent, with the exception of the ClaudeProcess (1000 atm Pressure), which gives a 45% per pass conversion.Obviously, these processes are very expensive in terms of energyconsumption. Now that energy is becoming increasingly expensive, it hasbecome important to search for catalysts which will do the ammoniasynthesis at lower temperatures and pressures.

As known, heat is evolved in ammonia synthesis. Thus the higher thetemperature the more the chemical equilibrium tends to shift, so tospeak, to the left. In other words, equilibrium is displaced to the leftby raising the temperature. To make possible the use of lowtemperatures, it is required that the catalyst have sufficient activityto effect reaction between hydrogen and nitrogen.

We have found a number of catalysts, which we have prepared, which areactive to catalyze the synthesis of ammonia from hydrogen and nitrogenat relatively lower temperatures.

Accordingly, it is an object of this invention to provide a process forthe synthesis of ammonia. Another object of the invention is to providea process for the synthesis of ammonia at relatively lower temperatures.A further object of the invention is to provide a catalyst which willsynthesize ammonia from hydrogen and nitrogen at relatively lowertemperatures.

Other aspects, concepts, objects, and several advantages of theinvention are apparent from study of this disclosure and the appendedclaims.

According to the present invention, there is provided a process for thesynthesis of ammonia from hydrogen and nitrogen in which there isemployed a composition comprising on a suitable support a salt selectedfrom at least one of an alkali metal, an alkaline earth metal, iron andcobalt hexacyanocobaltates and hexacyanoruthenates, the compositionhaving been heated in a manner to activate same for ammonia synthesis ata relatively lower temperature i.e., at a temperature in the range offrom about 725° F. to about 860° F.

Salts or compounds which have been found suitable to prepare thecomposition of the invention by adding these to a support and thenactivating include the following:

1. K₄ [Co(CN)₆ ] Potassium hexacyanocobaltate(II)

2. K₃ [Co(CN)₆ ] Potassium hexacyanocobaltate(III)

3. Ca₃ [Co(CN)₆ ]₂ Calcium hexacyanocobaltate(III)

4. Na₄ [Ru(CN)₆ ] Sodium hexacyanoruthenate(II)

5. K₄ [Ru(CN)₆ ] Potassium hexacyanoruthenate(II)

6. Rb₄ [Ru(CN)₆ ] Rubidium hexacyanoruthenate(II)

7. Cs₄ [Ru(CN)₆ ] Cesium hexacyanoruthenate(II)

8. Mg₂ [Ru(CN)₆ ] Magnesium hexacyanoruthenate(II)

9. Ca₂ [Ru(CN)₆ ] Calcium hexacyanoruthenate(II)

10. Ba₂ [Ru(CN)₆ ] Barium hexacyanoruthenate(II)

11. Fe₂ [Ru(CN)₆ ] Iron hexacyanoruthenate(II)

12. Co₂ [Ru(CN)₆ ] Cobalt hexacyanoruthenate(II)

As evident from a study of this disclosure some of the foregoingcompounds have been known.

Thus catalysts that have been found to be particularly active for theformation of ammonia from its elements, are prepared employing on asuitable support to produce a composition which is later activated saltsof cyano complexes of cobalt and ruthenium; in particular salts ofgroups Ia and IIa of the periodic table of the elements with bothtransition metals and, with a cyano complex of ruthenium, a cobalt salt.Mixtures of the salts are included as well as use of both the cobalt andruthenium in preparing the salts.

Catalysts are prepared from said salts supported on refractory oxidessuch as activated alumina, silica gel, titania, magnesia, zirconia, zinctitanate, kieselguhr, pumice, and the like, wherein an extended surfaceis available to increase the effectiveness of the catalyst.

Cyano complexes can be applied to the support by impregnation with asolution in which they are dissolved. Water is a suitable solvent forsome of the salts listed above. If the complex is relatively insolublein available solvents it can be applied to the support by precipitatingit from solutions that contain appropriate ions, i.e., a solution of thedesired cation and a solution of hexacyanocobalt or hexacyanoruthenium,by combining the solutions in the presence of the support. Neither ofthese solutions should contain compounds of sulfur or phosphorus becausethey reduce the activity of the resulting catalyst. It is preferablethat they do not contain halogen compounds which are generally corrosiveto process equipment. When the cyano complex is made by precipitation asdescribed here it is not necessary to separate the solid phase(precipitate and support) from the liquid phase.

After the cyano complex and the support have been combined, either byimpregnation or by precipitation, solvent is removed by evaporation andthe residual solid is usually converted to the active catalytic form byheating in an atmosphere comprising hydrogen. Some of the hydrogen maybe replaced by a material or gas which produces a desired activating orinert atmosphere. Heating must attain a temperature at least hot enoughto decompose the cyano complex and remove essentially all of thecontained nitrogen as determined by x-ray photoelectron spectroscopy. Itcan be to at least 475° C. Preferably, however, the catalyst is heatedto about 325°-430° C. for a time sufficient to produce an activecatalyst. This may be from 0.1 to 10 hours, or even longer. Generally atime of 0.5 to 2 hours is suitable.

The chemical nature of the activated catalyst is not known; althoughessentially all of the nitrogen has been removed it is not known whetherthe accompanying carbon is also removed during activation. In additionthe form of the Group Ia or IIa elements after activation is alsounknown. And, when the cyano complex is formed by precipitation, otherelements that were present initially will also affect the nature of theactivated catalyst. Because of this uncertainty the concentration ofcatalyst on support cannot be defined in terms of compounds actuallypresent. However, the composition that comprises the cyano complex plusthe support, before activation but calculated on an anhydrous (orsolvent-free) basis, should contain between about one to about 15 weightpercent of cobalt and/or ruthenium, at least one of which is present asthe hexacyano complex.

Process

Mixtures of nitrogen and hydrogen are used to synthesize ammonia. Thestoichiometric composition of 3 hydrogen:nitrogen is preferred, but thecomposition can range from 0.3-30 volumes of hydrogen per volume ofnitrogen.

The temperature at which ammonia can be made can range between about260°-590° C. However, preferably, according to the invention, thetemperature will be between about 385°-450° C. This is lower,significantly lower, than that of present commercial technology.

Ammonia synthesis can be effected at pressures between about 10⁵ to 10⁸Pa. Preferably reactant pressure will range between about 2×10⁶ to 8×10⁶Pa.

Contact time of reactants with the catalyst, expressed as volumes of gasat standard conditions per unit volume of catalyst per hour (GHSV) canrange from as low as 100 to about 5000. Preferably the gas hourly spacevelocity will be between about 500-2000.

Preparation of nine different catalysts illustrative of this invention,together with an iron-containing catalyst, and results from runs inwhich they were used to synthesize ammonia, are described in thefollowing examples.

EXAMPLE I

Catalysts prepared by impregnation. Catalyst A was prepared by adding asolution of 3.43 g K₄ [Ru(CN)₆ ].3 H₂ O (from Alfa division of VentronCorp.) in 9.0 mL of water to 8.0 g (21 mL) of silica gel. The gelabsorbed nearly all the solution. This was dried in an oven for 2.5 hr.at 110° C.

Catalyst B was prepared by adding a solution of 3.5 g Ca₂ [Fe(CN)₆ ].xH₂ O (from Henry Bower Chem. Mfg. Co.) in 9.0 mL of water to 8.0 g ofsilica gel. This was dried in an oven for 4 hr. at 110° C.

Catalyst C was prepared by adding a solution of 3.5 g of K₄ [Ru(CN)₆ ].3H₂ O in 11 mL of water to 15.7 g of -10+40 mesh Harshaw alumina having asurface area of about 200 m² /g. This was dried in an oven for 3 hr. at110° C.

Catalyst D was prepared by adding a solution of 3.66 g of K₃ [Co(CN)₆ ](from Pfaltz and Bauer) in 9.0 mL of water to 12.2 g of the same kind ofalumina that was used to make catalyst C. This was dried in an oven for4 hr. at 110° C.

Catalyst E was prepared by adding 13 mL of a solution that wascalculated to contain 104 g of calcium hexacyanocobaltate(III) per literof solution to 18.0 g of -10+40 mesh Norton alumina having a surfacearea of about 20 m² /g. (The solution of calcium hexacyanocobaltate wasprepared by neutralizing the appropriate acid with calcium carbonate asfollows. A column containing 51 g of the hydrogen form of cationexchange resin ANGC-244 (from J. T. Baker Chem. Co.) was washed withdistilled water until the effluent was neutral to pH indicator paper. Asolution containing 12.5 g of K₃ [Co(CN)₆ ] in 125 mL of water waspoured through the resin in about 15 minutes; the column was washed with65 mL water which was added to original eluent. About 7.7 g of calciumcarbonate was added to this solution; vigorous evolution of CO₂followed. After warming and stirring for about an hour the undissolvedcalcium carbonate was removed by filtration; it weighed 2.0 g afterdrying. The solution was further evaporated to 100 mL total volume.).After stirring to distribute the liquid the mixture was dried in an ovenfor one hour at 105° C.

Catalyst F was prepared just as catalyst E was but contained about threetimes as much metal. 12.0 mL of the Ca₃ [Co(CN)₆ ]₂ solution describedabove was added to 17.0 g of the same kind of alumina that was used tomake catalyst E. The mixture was dried in an oven for 1.5 hr. at 105° C.The process of adding cobalt solution and drying was repeated twicemore.

EXAMPLE II

Catalysts prepared by acid neutralization, then impregnation.

Three catalysts were prepared from hexacyanoruthenic acid made asfollows. A column containing 13.5 g of the hydrogen form of cationexchange resin ANGC-244 was washed with water until the effluent wasneutral. A solution of 3.0 g K₄ [Ru(CN)₆ ].3H₂ O in 10 ml of water wasapplied to the column. The column was rinsed with water until eluent wasno longer strongly acidic. About 50 mL of eluent containinghexacyanoruthenic acid was collected. It was divided into three equalparts and used to make catalysts G, H, and J.

Catalyst G was prepared by adding 0.5 g CaCO₃ to the solution of acid.After effervescence had ceased this solution was added to 11 g of silicagel. This was then dried in an oven.

Catalyst H was prepared by adding 1.4 g Ba(OH)₂.8H₂ O to the solution ofacid. This was sufficient to make the solution basic; it was poured over11 g of silica gel, mixed, and dried in an oven.

Catalyst J was prepared by adding 0.5 g CaCO₃ to the solution of acid.After effervescence had ceased this solution was added to 17 g of thesame kind of alumina that was used to make catalyst E. This was thendried in an oven.

EXAMPLE III

Catalyst prepared by precipitation onto a support.

Catalyst K was prepared as follows. To a solution of 3.4 g (0.0137moles) of Co(C₂ H₃ O₂)₂.4H₂ O in 50 mL water containing 0.7 mL glacialacetic acid 20 g of powdered Catapal alumina, having a surface area of268 m² /g, was added. A solution containing 3.2 g (0.0068 moles) of K₄[Ru(CN)₆ ].3H₂ O in 30 mL of water was added to the cobaltacetate-alumina slurry, then 1.3 mL glacial acetic was added. Thisproduced a smooth slurry that set to a firm gel within a few minutes.This gel was dried in an oven overnight at 110° C.

EXAMPLE IV

All 10 catalysts whose preparation has been described in Examples I-IIIwere used in runs to make ammonia from a hydrogen-nitrogen mixture.These were made using a 1/2-inch schedule 40 stainless steel pipereactor containing 22.5 mL of catalyst. The reactor was heated in atemperature controlled electric furnace. All runs were made at 1.76×10⁶Pa (255 psia) using a premixed feed containing 50 mole percent each ofhydrogen and nitrogen. After placing the indicated volume of catalyst inthe reactor it was heated to about 425° C. in a stream of pure hydrogenfor about an hour at 1.76×10⁶ Pa. Hydrogen was replaced with the ammoniasynthesis gas and, after reactor temperature and pressure had beenestablished an approximately one hour run was made in which reactoreffluent was contacted with a solution of standardized acid to absorbammonia. At the conclusion of the run excess acid was back-titrated withstandard caustic using brom cresol green indicator. Effluent from theacid scrubber was measured with a wet test gas meter, then vented. Totaleffluent volume was the sum of this observed volume and the calculatedvolume of the ammonia removed in the scrubber. Table I contains resultsfrom runs with these catalysts. It presents the concentration of ammoniagas in the reactor effluent and also expresses this as "Percent ofEquilibrium" that were calculated from chemical reaction equilibriumconstants from the International Critical Tables, 7, 239. Table I alsocontains the temperature and feed rate for the run, and shows theconcentration of the Group VIII element (Fe, Co, Ru) calculated on thebasis of the anhydrous catalyst before activation in hydrogen atelevated temperature.

                  TABLE I                                                         ______________________________________                                        Group VIII                Ammonia                                                    Element,   Temp.,        Mole % in                                                                             % of                                  Catalyst                                                                             Wt. %      °C.                                                                            GHSV  Product Equil.                                ______________________________________                                        A            Ru, 6.48 430    989  3.58    110.                                B            Fe, 3.95 428   1111  1.27    38.0                                C            Ru, 3.96 428   1008  3.52    105.                                D            Co, 4.10 428   1045  1.82    54.2                                E            Co, 1.50 429   1114  1.13    34.2                                F            Co, 3.85 436   1200  3.10    103.                                G            Ru, 2.64 433   1174  1.83    57.9                                H            Ru, 1.79 429   1214  2.50    75.8                                J            Ru, 1.70 427   1140  2.51    73.8                                K            Co, 3.55 433   1059  1.02    32.3                                             Ru, 3.06                                                         ______________________________________                                    

Catalyst B is active for ammonia synthesis but produced a smaller yieldthan all catalysts except E and K. Catalyst E contains less than halfthe concentration of Group VIII metal compared to catalyst B. It is notknown why the combination of two active elements--cobalt andruthenium--in catalyst K did not exhibit higher activity. These runsshow that silica and three different aluminas are suitable supports forthe catalysts of this invention.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure and the appended claims to the invention theessence of which is that compositions prepared from certain salts asdescribed, some of them not heretofore known, have been prepared byadding the salts to a suitable support and then heating the compositionthus obtained under conditions suitable to produce an ammonia synthesiscatalyst which is effective at relatively lower temperatures than thoseof presently known commercial technology about 900° F. to 950° F.; andthat a process employing such catalysts at a temperature from about 725°F. to about 860° F. has been set forth.

We claim:
 1. A composition which can be heated at a temperature withinthe approximate range 325°-430° C. to activate the same to render iteffective for ammonia synthesis at a relatively low temperature in therange 385°-460° C. which comprises on a suitable support at least onesalt selected from the group consisting of an alkali metalhexacyanocobaltate, an alkaline earth metal hexacyanocobaltate, analkali metal hexacyanoruthenate, an alkaline earth metalhexacyanoruthenate, iron hexacyanocobaltate, iron hexacyanoruthenate,cobalt hexacyanocobaltate and cobalt hexacyanoruthenate.
 2. Acomposition according to claim 1 wherein the support is a refractoryoxide.
 3. A composition according to claim 2 wherein the refractoryoxide is selected from the group consisting of activated alumina, silicagel, titania, magnesia, zirconia, zinc titanate, kieselguhr and pumice,each of said oxides having an extended surface available to increaseeffectiveness of the ultimate catalyst.
 4. A catalyst compositionaccording to claim 1 wherein the composition has been heated in anatmosphere of hydrogen until essentially all of the nitrogen has beenremoved from the composition.
 5. A process for preparing a catalysteffective for ammonia synthesis from nitrogen and hydrogen whichcomprises heating a composition according to claim 1 in an atmosphere ofhydrogen until essentially all of the nitrogen has been removed from thecomposition.
 6. As a new compound a salt selected from the groupconsisting of the following:Ca₃ [Co(CN)₆ ]₂ Calciumhexacyanocobaltate(III) Na₄ [Ru(CN)₆ ] Sodium hexacyanoruthenate(II) Rb₄[Ru(CN)₆ ] Rubidium hexacyanoruthenate(II) Cs₄ [Ru(CN)₆ ] Cesiumhexacyanoruthenate(II) Mg₂ [Ru(CN)₆ ] Magnesium hexacyanoruthenate(II)Ca₂ [Ru(CN)₆ ] Calcium hexacyanoruthenate(II) Ba₂ [Ru(CN)₆ ] Bariumhexacyanoruthenate(II) Fe₂ [Ru(CN)₆ ] Iron hexacyanoruthenate(II) Co₂[Ru(CN)₆ ] Cobalt hexacyanoruthenate(II).